Not Applicable.
Not Applicable.
This invention relates to a method and apparatus for the continuous casting of metals, and particularly the casting of metal strip. The continuous casting of thin metal strip has been employed with increasing success. The conventional twin-belt caster is employed to cast in widths up to 80 inches, but typically 0.75 inch thick, requiring three in-line rolling mill stands to produce coils with strip 0.1 inches thick.
In heat sink thin strip casters, such as disclosed in U.S. Pat. Nos. 5,564,491, 5,515,908, and 6,044,896 and World Patents WO 09517274A1 and WO 9714520A, also using twin-belts, the thickness cast is typically 0.1 inch. The ability to cast wider than 20 inches with this technology, however, is unproven. In the prior art heat sink belt casters, the molten metal is fed to the curved portion of the belts on the entry pulleys, and solidification of the metal is complete by the nip of the belts on the entry pulleys. The gap between entry pulleys is adjusted to create sufficient force to cause some elongation of the strip. In adjusting the gap force, using the apparatus described in U.S. Pat. No. 6,044,896, the horizontal distance to the nozzle tip from the top pulley is adjusted at the same time as the vertical distance between top and bottom pulleys. The belts are cooled in the return loop where the belts are not in contact with molten or solid strip. The cast gauge, about 0.1 inch, is the same as that obtained with conventional twin-belt casters after three rolling passes. In the prior art heat sink casters, side dams are located before the nip of the entry pulleys by means of a combination of stationary mechanical and electromagnetic edge dams. One example of such edge dams is shown in World Patent , WO 98/36861. The solidification rate is semi-rapid, which is a metallurgical advantage for many products, but unsuitable for making can body stock requiring galling resistance. Typically, with prior art heat sink belt caster operations, after three rolling mill stands the strip thickness is down to 0.01 inch.
In conventional twin-belt strip casting equipment, two moving belts are provided which define between them a moving mold for the metal to be cast. Revolving mechanical side dam blocks fill the gap between the belts in the molding section, which necessitates that the belts be parallel in the molding section. Such parallel belts mandate that the thickness of the cast product will be nearly the same as the height of the tip delivering molten metal. Cooling of the belts is typically effected by contacting a cooling fluid with the side of the belt opposite the side in contact with the molten metal. As a result, the belt is subjected to extremely high thermal gradients, with solidifying metal in contact with the belt on one side and a water coolant in contact with the belt on the other side. The dynamically unstable thermal gradients cause distortion in the belt, and consequently neither the upper nor the lower belt is flat without adding various devices to prevent areas of segregation and porosity. The belts are more prone to distortion when the machine is wider.
Various improvements have been proposed in the prior art, including preheating of the belts as described in U.S. Pat. Nos. 3,937,270 and 4,002,197, continuously applied and removed parting layers as described in U.S. Pat. No. 3,795,269, moving endless side dams as described in U.S. Pat. No. 4,586,559 and improved belt cooling as described in U.S. Pat. Nos. 4,061,177, 4,061,178 and 4,193,440. These various improvements and others have helped the quality of the cast surface, but the cast thickness is too large to achieve important economies in the downstream rolling. Furthermore, good surface quality is more difficult to achieve as the width is increased.
Another continuous casting process that has been proposed in the prior art is that known as block casting. In that technique, a number of chilling blocks is mounted adjacent to each other on a pair of opposing tracks. Each set of chilling blocks rotates in the opposite direction to form therebetween a casting cavity into which a molten metal such as an aluminum alloy is introduced. The liquid metal in contact with the chilling blocks is cooled and solidified by the heat capacity of the chilling blocks themselves. Block casting thus differs both in concept and in execution from continuous belt casting. Block casting depends on the heat transfer, which can be effected by the chilling blocks. Thus, heat is transferred from the molten metal to the chilling blocks in the casting section of the equipment and then extracted on the return loop. Block casters thus require precise dimensional control to prevent flash (i.e. transverse metal fins) caused by small gaps between the blocks. Such flash causes sliver defects when the strip is hot rolled. As a result, good surface quality is difficult to maintain. Examples of such block casting processes are set forth in U.S. Pat. Nos. 4,235,646 and 4,238,248.
Another technique, which has been proposed in continuous strip casting, is the single drum caster. In single drum casters, a supply of molten metal is delivered to the surface of a rotating drum, which is internally water cooled, and the molten metal is dragged onto the surface of the drum to form a thin strip of metal which is cooled on contact with the surface of the drum. The strip is frequently too thin for many applications, and the free surface has poor quality by reason of slow cooling and micro-shrinkage cracks. Various improvements in such drum casters have been proposed. For example, U.S. Pat. Nos. 4,793,400 and 4,945,974 suggest grooving of the drums to improve surface quality; U.S. Pat. No. 4,934,443 recommends a metal oxide on the drum surface to improve surface quality. Various other techniques are proposed in U.S. Pat. Nos. 4,979,557, 4,828,012, 4,940,077 and 4,955,429.
Another approach, which has been employed in the prior art, has been the use of twin drum casters, such as in U.S. Pat. Nos. 3,790,216, 4,054,173, 4,303,181, or 4,751,958. Such devices include a source of molten metal supplied to the space between a pair of counter-rotating, internally cooled drums. The twin drum casting approach differs from the other techniques described above in that the drums exert a compressive force on the solidified metal, and thus effect hot reduction of the alloy immediately after freezing. While twin drum casters have enjoyed the greatest extent of commercial utilization, they nonetheless suffer from serious disadvantages, not the least of which is an output typically ranging about 10% of that achieved in the prior art devices described above. Once again, the twin drum casting approach, while providing acceptable surface quality in the casting of high purity aluminum (e.g. foil), suffers from poor surface quality when used in the casting of aluminum with high alloy content and wide freezing range. Another problem encountered in the use of twin drum casters is centerline segregation of the alloy due to deformation during solidification. These machines have demonstrated the ability to make wide product, but the production rate is typically only 10% per unit of width of heat sink and conventional belt casters.
There is thus a need to provide an apparatus and method for continuously casting metallic strip at high speeds, thin thickness and wide widths as compared to methods currently employed.
It is accordingly an object of the present invention to provide an apparatus and method for continuously casting thin metallic strip (i.e. 0.1 inch thick) using conventional wide twin-belt casters that apply coolant to at least one belt in the molding section.
Another objective of the invention is to provide an apparatus and method for the continuous casting of thin metallic strip which permit the production of wide strip (i.e. up to 80 inches) on heat sink belt casters, while retaining the high speed and thin thickness, with no cooling applied in the molding section.
Another specific objective is to provide, in one machine, a range of solidification rates for different product requirements, including a slow rate for can body stock to provide galling resistance.
These and other objects and advantages of the invention appear more fully hereinafter from a detailed description of the invention.
The concepts of the present invention reside in a method and apparatus for continuous strip casting of metals utilizing a twin-belt strip casting approach in which the molding section between the belts is large at the point of molten metal entry and tapers to a smaller thickness part way through the length of the machine where a pair of pinch rolls sets the final thickness near the end of the molten metal sump. The pinch roll gap, pinch roll-separating force, and caster speed are regulated to provide the desired strip thickness. The pinch force serves to reduce cracking and to control the strip thickness profile across the width, which is critical for successful downstream rolling.
In the present invention, the molten metal is preferably applied to the belts after the nip of the entry pulleys. Because the belts converge toward one another in the molding section, conventional tip designs, which are thick, can be utilized for feeding molten metal into the machine, while making thin strip. Solidification takes place in the tapered molding section with the belts converging toward each other by means of a pair of pinch rolls located between the tip of the casting nozzle and the exit pulleys. The strip is solidified in the molding section, which extends from the molten metal entry point to the pinch rolls. There is a strip conveyance section extending from the pinch rolls to the exit pulleys.
In the preferred embodiment of the present invention, the heat sink capacity of the belts is used for solidifying the molten metal in the molding section, and the belts are cooled in the return loop where no solidification is occurring. In that way, the method and apparatus of the present invention minimize or avoid the erratic distortion effects caused by high non-uniform thermal gradients across twin-belt strip casters of the prior art. However, the tapering of the molding section does not preclude the use of applying cooling means on the opposite side of the belts in the molding section to make thicker product, if desired.
In the present invention, the containment of molten metal on the tapered edges, after the casting nozzle tip, can be accomplished by electromagnetic means. Alternatively, edge containment can be accomplished by mechanical edge dam blocks moving with the belts and sealing on the top of the bottom belts and the side edges of the top belts.
The belts utilized in the present invention can be provided with different coatings having different thermal resistances in order to provide rapid or slow solidification and short or long solidification lengths. Thus, by varying the coatings on the belts, the metallurgical structure can be varied depending on the needs of the product. For, example, slow solidification is desirable for making can body stock with good galling resistance.
The concepts of the present invention can be employed in the strip casting of most metals, including steel, copper, zinc and lead, but are particularly well suited to the casting of thin aluminum alloy strip, while overcoming the problems of the prior art.